Method for preparing polypropylene (pp) films having improved ultraviolet (uv) radiation stability and service life

ABSTRACT

Method for preparing stretched polypropylene films from a polypropylene resin that includes a low molecular weight hindered amine light stabilizer, and a high molecular weight hindered amine light stabilizer, and optionally a first antioxidant, and a second antioxidant. Also provided are methods for the preparation of woven jumbo polypropylene bags from the stretched polypropylene film.

RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 12/396,343, filed on Mar. 2, 2009, now issued asU.S. Pat. No. 7,947,768, and U.S. patent application Ser. No.13/106,679, filed on May 12, 2011, now issued as U.S. patent Ser. No.______, the disclosure of which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention generally relates to a polyolefin resin and articlesprepared from the polyolefin resin. More specifically, the inventionrelates to a polypropylene resin exhibiting improved ultraviolet (UV)radiation stability and articles prepared therefrom.

2. Description of the Prior Art

Polyolefin resins are used in a variety of applications due to the easeof use in the manufacturing of a variety of extruded and moldedarticles. Jumbo bags are an exemplary article that may be prepared frompolyolefin resins, which are useful in a variety of applications,including the transport, handling, storage and packaging of bulkmaterials, including materials having a lumpy or fine consistency. Thesearticles can be used in the transport and storage of a variety ofmaterials, including bulk quantities of petrochemical products,fertilizers, cement, sand, minerals, grains, feeds, seeds, mineralsalts, solid chemicals, sugar, and the like. The bags can be used tocarry loads up to 3 tons.

However, jumbo bags prepared from polypropylene resins frequently sufferfrom loss of the tensile strength and general instability due toexposure to UV radiation, particularly in outdoor applications where thebags may be exposed to prolonged sunlight. As the tensile strength ofthe bags decreases, the failure rate of the bags increases, therebydecreasing the service life of the bags and reducing the overallusefulness of the bags.

Thus, there exists a need for the development of a polyolefin resin,particularly a polypropylene resin, and articles prepared therefrom,having improved UV stability and increased lifetimes.

SUMMARY OF THE INVENTION

The invention provides a polypropylene resin for the preparation ofarticles having improved UV stability. Also provided are methods ofmanufacturing the same.

In one aspect, a stretched polypropylene film is provided. The filmincludes a polypropylene film that includes a polypropylene resin, thepolypropylene resin including polypropylene, a low molecular weighthindered amine light stabilizer; and a high molecular weight hinderedamine light stabilizer. The polypropylene film is stretched to a ratioof between approximately 1:7 and 1:5 to produce the stretchedpolypropylene film.

In another aspect, a method for preparing a stretched polypropylene filmis provided. The method includes the steps of extruding a polypropyleneresin to form a polypropylene film; and stretching the polypropylenefilm to form the stretched polypropylene film. The polypropylene filmincludes a polypropylene polymer, a low molecular weight hindered aminelight stabilizer; and a high molecular weight hindered amine lightstabilizer.

In another aspect, a stretched polypropylene film is provided. Thestretched polypropylene film includes a polypropylene resin, thatincludes polypropylene, a low molecular weight hindered amine lightstabilizer, a high molecular weight hindered amine light stabilizer, afirst antioxidant, and a second antioxidant. The polypropylene film isstretched to a ratio of between approximately 1:7 and 1:5 to produce thestretched polypropylene film.

In certain embodiments, the low molecular weight hindered amine lightstabilizer, the high molecular weight hindered amine light stabilizer,the first antioxidant, the second antioxidant, and any other additivemake up less than about 1% of the polypropylene resin by weight. Incertain other embodiments, the polypropylene resin is extruded toproduce a polypropylene film, and the polypropylene film is stretched ata ratio of between about 1:5 and 1:9 to produce polypropylene tapes. Incertain embodiments, the polypropylene film can be woven to prepare thewoven polypropylene bags. In certain other embodiments, thepolypropylene resin is extruded into a film at a temperature of betweenabout 220° and 250° C. In certain other embodiments, the polypropyleneresin further includes calcium stearate. In certain embodiments, thestretched polypropylene films can be woven to prepare jumbopolypropylene bags, and the bags can include a polymer liner.

In certain other embodiments, the polypropylene resin can include atleast two antioxidants, wherein the first antioxidant is a hinderedphenolic antioxidant and the second antioxidant is a phosphite ester.

In certain embodiments, the low molecular weight hindered amine lightstabilizer can be bis-(2,2,6,6-tetramethyl-4-piperidinyl)-decanedioate.In certain embodiments, the high molecular weight hindered amine lightstabilizer can bepoly[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl-4-piperidyl)imino]-hexamethylene-[(2,2,6,6-tetramethyl-4-piperidyl)imino]].

In another aspect, a method for preparing a stretched polypropylene filmis provided. The method includes the steps of extruding a polypropyleneresin to form a polypropylene film. The polypropylene resin includes apolypropylene polymer, a low molecular weight hindered amine lightstabilizer, a high molecular weight hindered amine light stabilizer, afirst antioxidant, and a second antioxidant. The polypropylene resin isstretched to form the stretched polypropylene film.

In certain embodiments, the ratio of low molecular weight hindered aminelight stabilizer to high molecular weight hindered amine lightstabilizer is between about 1:5 and 1:9. In certain embodiments, theratio of low molecular weight hindered amine light stabilizer to highmolecular weight hindered amine light stabilizer is about 1:7. Incertain embodiments, the low molecular weight hindered amine lightstabilizer, the high molecular weight hindered amine light stabilizer,the primary antioxidant and the secondary antioxidant are included inthe polypropylene resin in a total amount of less than about 1% byweight of the polypropylene resin. In certain embodiments, thepolypropylene resin is extruded at a temperature of between about 220°and 250° C. to form stretched polypropylene films. In certainembodiments, a stretched polypropylene film is prepared by stretchingthe polypropylene film at a ratio of about 1:7.

In certain other embodiments, the polypropylene resin can include atleast two antioxidants, wherein the first antioxidant is a hinderedphenolic antioxidant and the second antioxidant is a phosphite ester.

In certain embodiments, the low molecular weight hindered amine lightstabilizer can be bis-(2,2,6,6-tetramethyl-4-piperidinyl)-decanedioate.In certain embodiments, the high molecular weight hindered amine lightstabilizer can bepoly[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl-4-piperidyl)imino]-hexamethylene-[(2,2,6,6-tetramethyl-4-piperidyl)imino]].

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing tensile strength retention of polypropylenefilms upon accelerated weathering.

FIG. 2 is a graph showing tensile strength retention of polypropylenefilms after natural weathering.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides polyolefin resins having additives whichprovide for increased UV stability. Increased UV stability allows forprolonged exposure to sunlight, and increased overall lifetimes ofproducts prepared from the resins disclosed herein. Particularly,preferred polyolefin resins include, for example, polypropylene, atleast one hindered amine light stabilizer and at least one antioxidant.Articles prepared from the polypropylene resin prepared as describedtypically exhibit increased UV stability and increased lifetime.

Hindered Amine Light Stabilizers

Polymers prepared according to the present invention can include atleast one hindered amine light stabilizer (hereinafter “HALS”).Typically, the HALS is added to the polypropylene resin and acts as aphoton scavenger. Typically, the HALS reacts with UV light from sunshineto protect the bulk polymer matrix from photodegradation. The damagingradiation is absorbed by these additives and is typically converted intoharmless heat.

The HALS can be oxidized to form a nitroxyl radical, which can thenscavenge alkyl radicals in the bulk polymer leading to production ofhydroxylamines. The hydroxylamines can in turn react with peroxylradicals and, in turn, regenerate nitroxyl radicals. The cycling ofnitroxyl radical formation is one benefit of using HALS, even at lowconcentrations. Most commercially available UV stabilizing HALS absorbUV radiation in the range of between about 300-360 nm, generallycorresponding to the spectral region of sunlight that is typicallyresponsible for causing substantial damages to polymers and articlesprepared from polymers.

In certain embodiments, the molecular weight of the HALS can impact theperformance of the resulting polypropylene compounds. The relativeimpact of the HALS on polypropylene films can depend on the sample formand exposure conditions. For example, in relatively thick polypropylenefilms where the protection of the polymer film surface is particularlyimportant, there is a reduced performance of the HALS as the molecularweight of the stabilizer increases from approximately 500 g/mol toapproximately 1000 g/mol. In contrast, for relatively thin polypropylenefilm samples the effect of the molecular weight of the HALS is lesspronounced for the particular film thickness, but is neverthelesssignificant.

In addition, increased molecular weight of the HALS can result indecreased volatility and decreased diffusion rate through apolypropylene film. As a result, the performance of the HALS can reach amaximum at some particular molecular mass and their performance willdecrease thereafter as the stabilizer mass increases beyond theparticular molecular weight.

The UV stability and thermo-oxidative stability of polypropylene filmswere examined for polypropylene films that include a low molecularweight HALS, a high molecular weight HALS, a first antioxidant and asecond antioxidant. Depending on the particular stabilizer, the UV andthereto-oxidative stabilities decreased as the molecular weight of theHALS increased. The decrease in UV and thermo-oxidative stabilitytypically begins with number average molecular weight of the HALS ofapproximately 1000 g/mol. In certain embodiments, HALS having molecularweights up to 2000 or 3000 g/mol show a decrease in the UV andthermo-oxidative stability. The contribution of polymeric HALS tothermo-oxidative stability decreases as the molecular weight of the HALSreaches very high values, such as for example, at a molecular weight ofgreater than 5000 g/mol. However, the contribution of the HALS to the UVstability does not decrease with increasing molecular weight, in partdue to the deactivation of catalyst residues or other species that mayinitiate UV degradation, but are inactive with respect to the initiationof thermo-oxidative degradation.

In certain preferred embodiments, the HALS useful for inclusion in thepolypropylene resins described herein includes at least one piperidinefunctional group. Hindered light amine stabilizers having a piperidinefunctional group can be of relatively low molecular weight or ofrelatively high molecular weight. As used herein, low molecular weightHALS include monomeric compounds (I.e., not oligomeric or polymeric)having a molecular weight of less than about 1000 g/mol, preferablybetween about 200 and 800 g/mol. As used herein, high molecular weightHALS include oligomeric or polymeric compounds having a molecular weightof greater than about 1000 g/mol, preferably greater than about 1500g/mol, and can be, in certain embodiments, up to about 20,000 g/mol.

In certain embodiments, the low molecular weight HALS can be selectedfrom 2,2,6,6-tetramethyl-4-piperidon; 2,2,6,6-tetramethyl-4-piperidinol;bis-(1,2,2,6,6-pentamethylpiperidyl)-(3′,5′-di-tert-butyl-4′-hydroxybenzyl)-butylmalonate;bis-(2,2,6,6-tetramethyl-4-piperidinyl)-decanedioate (Tinuvin® 770);bis-(2,2,6,6-tetramethyl-4-piperidinyl)-succinate (Tinuvin® 780);bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl)-sebacate (Tinuvin®123); bis-(1,2,2,6,6-pentamethyl-4-piperidinyl)-sebacate (Tinuvin® 765);tetrakis-(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate;N,N-bis-(2,2,6,6-tetramethyl-4-piperidyl)-hexane-1,6-diamine;N-butyl-2,2,6,6-tetramethyl-4-piperidinamine;5-(2,2,6,6-tetramethyl-4-piperidinyl)-2-cyclo-undecyl-oxazole)(Hostavin®. N20);1,1′-(1,2-ethane-di-yl)-bis-(3,3′,5,5′-tetramethyl-piperazinone)(Goodrite® UV3034);8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro(4,5)decaan-2,4-dione(Tinuvin® 440); 1,2,3,4-butane-tetracarboxylicacid-1,2,3-tris(1,2,2,6,6-pentamethyl-4-piperidinyl)-4-tridecylester(Mark® LA62); N-2,2,6,6-tetramethyl-4-piperidinyl-N-amino-oxamide(Lucheme HAR100); 4-acryloyloxy-1,2,2,6,6-pentamethyl-4-piperidine;mixture of esters from 2,2,6,6-tetramethyl-4-piperidinol and fatty acids(Cyasorb® UV3853); propanedioic acid, [(4-methoxyphenyl)methylene]-,bis(1,2,2,6,6-pentamethyl-4-piperidinyl)ester (Sanduvor® PR 31);formamide, N,N-1,6-hexanediylbis[N-(2,2,6,6-tetramethyl-4-piperidinyl(Uvinul® 4050H); 1,5-dioxaspiro (5,5) undecane 3,3-dicarboxylic acid,bis(2,2,6,6-tetramethyl-4-peridinyl) ester (Cyasorb® UV-500);1,5-dioxaspiro (5,5) undecane 3,3-dicarboxylic acid,bis(1,2,2,6,6-pentamethyl-4-peridinyl)ester (Cyasorb® UV516);3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidyl)-pyrrolidin-2,5-dione(Cyasorb® UV3581); and3-dodecyl-1-(1,2,2,6,6-pentamethyl-4-piperidyl)-pyrrolidin-2,5-dione. Incertain embodiments, the low molecular weight hindered amine lightstabilizer includes at least one piperidine functional group, and morepreferably the hindered amine light stabilizer includes at least onepolyalkyl substituted piperidine functional group, and even morepreferably the hindered amine light stabilizer isbis-(2,2,6,6-tetramethyl-4-piperidinyl)-decanedioate (Tinuvin® 770).

In certain embodiments, the high molecular weight HALS is selected from1,3,5-triazine-2,4,6-triamine;N′,N″-(ethanediylbis-(4,6-bis-(butyl(1,2,2,6,6-pentamethyl-4-piperidinyl-amino)-1,3,5-triazine-2-yl)-iminopropanedil)-N′,N″-dibutyl-N′,N″-bis-(1,2,2,6,6-pentamethyl-4-piperidinyl);poly-methylpropyl-3-oxy-4-(2,2,6,6-tetramethyl-piperidinyl-siloxane;octa-decene-(N-(2,2,6,6,-tetramethylpiperidinyl-4-N-maleic imido oxalicacid diamide) copolymer; andpoly-(6-morpholine-S-triazine-2,4-diyl)-2,2,6,6-tetramethyl-4-piperidinyl)-hexa-methylene-2,2,6,6-tetramethyl-4-piperidinyl)-imino.Preferably, the high molecular weight hindered amine light stabilizer ispoly[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl-4-piperidyl)imino]-hexamethylene-[(2,2,6,6-tetramethyl-4-piperidyl)imino]](Chimassorb® 944).

In certain embodiments, the high molecular weight HALS has a molecularweight that is at least about two times greater than the molecularweight of the low molecular weight HALS. In certain other embodiments,the high molecular weight HALS has a molecular weight that is at leastabout three times greater than the molecular weight of the low molecularweight HALS

Antioxidant Additives

Antioxidants present in the polypropylene resin help to preventoxidation of products produced therefrom. Exemplary antioxidants caninclude sterically hindered phenols, phosphites and phosphorites. Incertain embodiments, at least two antioxidants can be included in thepolypropylene resin. In certain other embodiments, the polypropyleneresin can include two antioxidants. In certain embodiments, the firstantioxidant can be selected from a hindered phenolic antioxidant and thesecond antioxidant can be selected from phosphite ester antioxidant.

In certain embodiments, the antioxidant can be a phenolic basedantioxidant. Exemplary phenolic antioxidants can include alkylatedmonophenols, alkylthiomethylphenols, hydroquinones and alkylatedhydroquinones, tocopherols, hydroxylated thiodiphenyl ethers, alkylidenebisphenols, O-, N- and S-benzyl compounds, hydroxybenzylated malonates,hydroxybenzyl aromatic compounds, triazine compounds,benzylphosphonates, acylaminophenols, esters ofβ-(3,5-di-tert-butyl-4-hydroxyphenyl) propionic acid with monohydric orpolyhydric alcohols, esters ofβ-(5-tert-butyl-4-hydroxy-3-methylphenyl)propionic acid with monohydricor polyhydric alcohols, esters ofβ-(3,5-dicyclohexyl-4-hydroxyphenyl)propionic acid with monohydric orpolyhydric alcohols, esters of 3,5-di-tert-butyl-4-hydroxyphenylaceticacid with monohydric or polyhydric alcohols, and amides ofβ-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid.

Specific antioxidants that may be used with the present compositioninclude: 2,6-di-t-butyl-4-methylphenol; 2,6-di-t-butyl-4-ethylphenol;octadecyl 3,5-di-t-butyl-4-hydroxy-hydrocinnamate (Irganox® 1076);benzenepropanoic acid, 3,5-bis(11,1-dimethylethyl)-4-hydroxy-methylester (Ralox 35); benzenepropanoic acid,3,5-bis(1,1-dimethylethyl)-4-hydroxy-isooctyl ester (Irganox® 1135);benzenepropanoic acid, 3,5-bis(1,1-dimethyl ethyl)-4-hydroxy-C₁₃₋₁₅branched and linear alkyl esters (Anox® BF); benzenepropanoic acid,3,5-bis(1,1-dimethylethyl)-4-hydroxy;2,2′-methylenebis(6-t-butyl-4-methylphenol) (Cyanox® 2246);2,2′-methylenebis 6-(1-methylcyclohexyl)-p-cresol (Lowinbx® WSP);4,4′-butylidenebis(6-t-butyl-3-methyl-phenol) (Santhowhite® powder);1,1,3-tris(2-methyl-4-hydroxy-5-t-butyl phenyl)butane (Topanol®) CA);N,N′-hexamethylene bis(3,5-di-t-butyl-4-hydroxyhydrocinnamamide(Irganox®) 1098); 2,2′-ethylidenebis(4,6-di-t-butylphenol) (Isonox®129); 4,4′-methylenebis(2,6-di-t-butylphenol) (Ethanox 702);tri-ethylene-glycol-bis-3-(t-butyl-4-hydroxy-5-methyl-phenyl)-propionate(Irganox® 245);1,6-hexane-diol-bis-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate(Irganox® 259); butylated hydroxyanisole (Teenox® BHA);2,6-di-t-butyl-4-sec-butyl-phenol (Isonox® 132);2,6-di-t-butyl-4-n-butyl-phenol; 2,6-di-t-butyl-4-nonyl-phenol (Isonox®232); 2,6-di-methyl-6-(1-methyl-cyclohexyl)-phenol (Lowinox® WSL);2,4-di-methyl-6-(1-methyl-6-(1-methyl pentadecyl)-2-propyleneacid,2-isopentane-6-[(3-isopentane-2-hydroxy-5-isopentane-phenyl)-ethyl]-4-methyl-phenyl-ester(Sumilizer® GS); 2-propylene-acid,2-t-butyl-6-[(3-t-butyl-2-hydroxy-5-methyl-phenyl)-methyl]4-methyl-phenyl-ester(Sumilizer® GM); di-ethyl-ester of3,5-di-t-butyl-4-hydroxy-benzyl-phosphoric acid (Irganox® 1222);2,5,7,8-tetra-methyl-2-(4′,8′,12′-tri-methyl-tri-decyl)-6-chromanol(Ronotec® 201);N,N′-1,3-propanediylbis(3,5-di-t-butyl-4-hydroxyhydrocinnamamide);calcium bis[monoethyl(3,5-di-t-butyl-4-hydroxy benzyl)phosphonate(Irganox® 1425).

In certain embodiments, the hindered phenolic antioxidant can bepentaerythritol tetrakis (3,5-di-tert-butyl-4-hydroxyhydrocinnamate). Incertain embodiments, the antioxidant can be octadecyl3,5-di-tert-butyl-4-hydroxyhydrocinnamate. In certain embodiments, thephosphite ester antioxidant can be tris-(2,4-di-tert-butylphenyl)phosphite.

As described herein, the polypropylene resin can include at least oneantioxidant and at least one HALS. The combination of antioxidants, HALSand phosphites can result in an overall synergistic effect, therebyincreasing the overall stability and lifetime of articles preparedtherefrom. For example, in certain embodiments, a low molecular weightHALS (e.g., a piperidine containing compound, such as, Tinuvin® 770) anda high molecular weight HALS (e.g., Chimassorb® 944) can be added to thepolypropylene resin. In certain embodiments, by using two or more HALS,the morphological differences of the HALS associated with polymeric,oligomeric and synergistic HALS can result in slightly differentpathways to protective reactions against UV damage. This may be linkedwith rate of interactions and diffusion mechanism of the individual HALSwithin the resin matrices under various compounding and processingconditions.

In certain embodiments, the total amount of additives added to thepolymer resin is less than about 5% by weight. In certain embodiments,the total amount of additives is less than about 3% by weight. Incertain embodiments, the total amount of additives is less than about 2%by weight. In certain embodiments, the total amount of additives is lessthan about 1% by weight.

In certain embodiments, the low molecular weight HALS can be present inan amount of between about 0 and 1% by weight. Preferably the lowmolecular weight HALS is present in an amount of between about 0 and0.2% by weight, between about 0.2 and 0.4% by weight, between about 0.4and 0.6% by weight, between about 0.6 and 0.8% by weight, between about0.25 and 0.75% by weight, and between about 0.8 and 0.9% by weight.

In certain embodiments, the high molecular weight HALS can be present inan amount of between about 0 and 1% by weight. In certain embodiments,the high molecular weight HALS is present in an amount of between about0 and 0.2% by weight. In other embodiments, the high molecular weightHALS is present in an amount between about 0.2 and 0.4% by weight. Inother embodiments, the high molecular weight HALS is present in anamount between about 0.4 and 0.6% by weight. In other embodiments, thehigh molecular weight HALS is present in an amount between about 0.6 and0.8% by weight. In other embodiments, the high molecular weight HALS ispresent in an amount between about 0.25 and 0.75% by weight. In otherembodiments, the high molecular weight HALS is present in an amountbetween about 0.8 and 0.9% by weight. In certain embodiments, the highmolecular weight HALS is present in an amount less than about 0.25% byweight. In other embodiments, the high molecular weight HALS is presentin an amount less than about 0.2% by weight. In other embodiments, thehigh molecular weight HALS is present in an amount less than about 0.15%by weight. In other embodiments, the high molecular weight HALS ispresent in an amount less than about 0.1% by weight.

In certain embodiments, the low molecular weight HALS can be present inan amount at least about 3 times greater than the amount of highmolecular weight HALS. In certain embodiments, the low molecular weightHALS: high molecular weight HALS ratio can be at least about 5, at leastabout 6, or at least about 7. In certain embodiments, the low molecularweight HALS: high molecular weight HALS ratio is between about 5 and 10.In certain embodiments, the low molecular weight HALS: high molecularweight HALS ratio is between about 5 and 7. In certain embodiments thelow molecular weight HALS: high molecular weight HALS ratio is about 7.

In certain embodiments, the first antioxidant can be present in anamount of between about 0 and 1% by weight. In other embodiments, thefirst antioxidant is present in an amount of between about 0 and 0.5% byweight. In yet other embodiments, the first antioxidant is present in anamount between about 0 and 0.1% by weight.

In certain embodiments, the second antioxidant can be present in anamount of between about 0 and 1% by weight. In other embodiments, thesecond antioxidant is present in an amount of between about 0 and 0.5%by weight. In yet other embodiments, the second antioxidant is presentin an amount between about 0 and 0.1% by weight.

In certain embodiments, calcium stearate, or a like compound, can bepresent in an amount between about 0 and 1% by weight. More preferably,calcium stearate can be present in an amount between about 0 and 0.5% byweight. In other embodiments, calcium stearate can be present in anamount between about 0 and 0.1% by weight.

The polypropylene employed in the various embodiments disclosed can beprepared by any known polymerization technique and with any knownpolymerization catalyst system. The polymerization reaction can besolution or gas phase polymerizations. Exemplary catalysts can includeZiegler-Natta, metallocene, or (other) single-site catalyst systems. Alltechniques are known in the art.

The polypropylene resin compositions according to the variousembodiments of the present invention can also include known additivesand fillers, in addition to the stabilizers and antioxidants disclosedherein. Exemplary additives can include fibers, fillers, lubricants,flame retarding agents, additional rubber(s), etc.

The polypropylene resin compositions according to the variousembodiments of the invention can be transformed into shaped(semi-)finished articles using a variety of processing techniques.Exemplary processing techniques include, but are not limited to,injection molding, injection compression molding, in-mould decoratingvia injection molding, extrusion, and extrusion compression molding.

Preparation of Films from the Polypropylene Resins

Typically, polypropylene films produced from resins having a broadmolecular weight distribution have a tensile strength of about 35 MPa.However, when these films are stretched at a ratio of about 1:7, thetensile strength is improved to greater than about 250 MPa, preferablygreater than about 300 MPa, and even more preferably greater than about350 MPa. In certain embodiments, the tensile strength of thepolypropylene film is increased by at least a factor of about 5 uponstretching, more preferably by at least a factor of about 7. Even morepreferably, upon stretching of the polypropylene film, tensile strengthis increased by at least a factor of about 10. In certain embodiments,the increased tensile strength can be partially due to the fact thatwhen the polymer film is stretched, the crystalline region of thepolypropylene domain is increased at the expense of the amorphousregion.

Thus, polypropylene bags prepared from the polypropylene resinsdescribed herein that include a low molecular weight HALS, a highmolecular weight HALS and at least two antioxidants demonstrate improvedultraviolet (LTV) stability, as compared with polypropylene films thatdo not include the combination of low and high molecular weight HALS andantioxidants. Typically, the polypropylene resin is stretched to a ratioof at least 1:5 times the original length, preferably about 1:7 timesthe original length.

After stretching, polypropylene films generally increase in crystallinephase. Thus, upon stretching, the overall amorphous phase present in thepolypropylene film is reduced relative to before stretching. As aresult, after stretching, the polypropylene films become stronger andhave an increased tensile strength. The polypropylene films describedherein, having a low and high molecular weight hindered amine lightstabilizer and at least two antioxidants, showed an increased tensilestrength to approximately 300 MPa after stretching, compared with atensile strength of about 35 MPa prior to stretching.

Examples

Basic resin polypropylene (PP 500P, coded as A) was obtained from SaudiArabian Basic Industries Co. (SABIC) and used without furtherpurification. Antioxidants (e.g., Irganox® 1010 and Irgafos® 168) andHALS Tinuvin® 770 (low molecular weight) and Chimassorb® 944 (highmolecular weight) were obtained from Ciba Speciality Chemicals Co., andused as received. Calcium stearate having a melting point ranging from140° C.-160° C., a bulk density 250-310 g/ml and 325 mesh screen wasobtained from Esterchem (M) Sdn. Bhd., and used as received.

As shown in Table 1, various exemplary formulations were developed withpolypropylene granular resin (PP 500P) in combination with variousadditives, which were mainly present in powder form and present in anamount of less than 1% of the bulk polymer matrix and are provided inTable 1. A master-batch technique was adopted by mixing about 5 g of themaster-batch material to about 100 g of the polypropylene resin toensure that an appropriate concentration of each desired additive wasobtained in the desired formulations at 5 phr basis.

Master-Batch Technique

A carrier LLDPE (linear low density polyethylene) powder, having adensity of 0.9182 g/cm³ and MFI (melt flow index) of 0.86 g/10 min wasused to make the master-batch. All additives were mixed together withthe carrier LLDPE powder resin for half an hour in a high-speed mixer(Model FM 10823 of Mitsui Mining Co. Ltd., Tochigi Engineering Works,Japan) at about 500 rpm and then pelletized by extruding the mixture ata temperature of between about 200° C.-230° C. with an extruder (modelL/D 19/25) attached to a Plasticoder (model PL2200), a rod die and apelletizer from Brabender Co., Germany. A polypropylene resin (PP 500P)was mixed with these master-batch pellets at about 5 phr in a Pascal LabMixer (model 1664-00-A, UK) for about one hour at about 50 rpm and thenpelletized at a temperature of between about 200° C.-230° C. with thesame extruder, rod die and the pelletizer. These pellets were then usedfor film preparation.

TABLE 1 Formulations with PP500P resin and additives. Antioxidants HALSFilm Irganox ® Irgafos ® Calcium Tinuvin ® Chimassorb ® Total code 1010(%) 168 (%) Stearate (%) 770 (%) 944 (%) Additive A — — — — — 0 A1 0.050.05 0.10 0.175 0.025 0.4% wt. A1/1 0.05 0.05 0.10 0.35 0.05 0.6% wt.A1/2 0.05 0.05 0.10 0.525 0.075 0.8% wt. A1/3 0.05 0.05 0.10 0.70 0.101.0% wt.

Film Preparation:

Films (0.450 mm thick) were prepared using the same extruder and aribbon die at a temperature of between about 220° C. and 250° C. bydrawing the melt mixture of the ribbon die through a set of steel drumthermo-stated at about 50° C. These films were stretched at atemperature of about 160° C. to approximately a 1:7 ratio by usingStretching Unit (model Teach-line) of Dr. Collin GmbH Co., Germany. Thestretched films were characterized and subjected to acceleratedweathering by QUV and natural (outdoor) weathering tests.

Accelerated Weathering by QUV:

This test was conducted for stretched polypropylene films by using a QUVWeathering instrument of Q-UV Panel Lab Products Co., Cleveland, Ohio,USA (model number QUV/Basic). The QUV instrument contains eightfluorescent lamps (model UVA-340) and conforms to ASTM D-4329 standards(standard practice for fluorescent UV exposure of plastics)corresponding to ISO 4892. Samples were removed from the instrument atregular intervals and the tensile strength (TS) of the samples weremeasured as per ASTM D368 M using a Universal testing machine (model4505) from Instron, UK. The gauge length of the samples was 2.5 cm andtest speed was 50 cm/min. TS retention (%) was evaluated for each sampleafter each exposure and the test was continued until a retention of 50%of the tensile strength was measured.

Natural (Outdoor) Weathering:

Stretched polypropylene films were cut into dumbbell shapes and mountedon plastic plates (measuring about 12 cm by 8 cm), and were placed onwooden boards positioned at an approximately 45° angle facingsouthwards. Samples were removed periodically and their tensileproperties were tested, according to the procedures described above,until a retention of 50% of the tensile strength was measured.

As shown in FIGS. 1 and 2, most of the samples demonstrate a highresistance to UV degradation under accelerated weathering by a Q-UVmachine and natural (outdoor) weathering tests.

As shown in FIG. 1, after exposure to UV radiation for approximately5500 hours in a Q-UV machine (which is roughly equivalent toapproximately 2 years of natural exposure to sunlight), tensile strengthretention of a stretched polypropylene film having between about 0.4 and1% by weight additives was approximately 87%. In comparison, thepolypropylene sample having no additives immediately demonstrated areduction in tensile strength retention.

As shown in FIG. 2, the tensile strength of a stretched polypropylenefilm after approximately 8 months of natural outdoor exposure exhibiteda tensile strength retention of approximately 112%. In contrast, similarto FIG. 1, the polypropylene films with no additives demonstrated animmediate decrease in tensile strength retention, reaching a retentionof about 50% in approximately 2 months.

The tensile strength retention (%) is observed around about 100% in theboth the tests.

While the present invention has been described with respect topolypropylene resins and articles prepared therefrom, particularly wovenjumbo bags, it is understood that the additives described herein and theprinciples discussed with respect to improved wear resistance, increasedUV and thermoxidative stability can also be applied to polymercompositions prepared from other polymer resins, such as for example,but not limited to, polyethylene, polyacetylene, polybutylene and otherpolyolefins, and various copolymers and blends thereof.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

As used herein, recitation of the term about and approximately withrespect to a range of values should be interpreted to include both theupper and lower end of the recited range.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

While the invention has been shown or described in only some of itsembodiments, it should be apparent to those skilled in the art that itis not so limited, but is susceptible to various changes withoutdeparting from the scope of the invention.

As used in the specification and claims, the singular form “a”, “an” and“the” may include plural references, unless the context clearly dictatesthe singular form.

Although the following detailed description contains many specificdetails for purposes of illustration, one of ordinary skill in the artwill appreciate that many variations and alterations to the followingdetails are within the scope of the invention. Accordingly, theexemplary embodiments of the invention described below are set forthwithout any loss of generality to, and without imposing limitationsthereon, the claimed invention. Reference throughout this specificationto “one embodiment,” “an embodiment,” or similar language means that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment,”“in an embodiment,” and similar language throughout this specificationmay, but do not necessarily, all refer to the same embodiment

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Throughout this application, where patents or publications arereferenced, the disclosures of these references in their entireties areintended to be incorporated by reference into this application, in orderto more fully describe the state of the art to which the inventionpertains, except when these references contradict the statements madeherein.

1. A method for preparing a stretched polypropylene film, the methodcomprising the steps of: extruding a polypropylene resin to form apolypropylene film; and stretching the polypropylene film to form thestretched polypropylene film; wherein the polypropylene film comprises:a polypropylene polymer; a low molecular weight hindered amine lightstabilizer; and a high molecular weight hindered amine light stabilizer.2. The method of claim 1 wherein the polypropylene film furthercomprises a first antioxidant.
 3. The method of claim 1 wherein thefirst antioxidant is a hindered phenolic antioxidant.
 4. The method ofclaim 1 wherein the first antioxidant is pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate).
 5. The method of claim 1wherein the first antioxidant is a phosphite ester.
 6. The method ofclaim 1 wherein the first antioxidant is tris-(2,4-di-tert-butylphenyl)phosphite.
 7. The method of claim 2 wherein the polypropylene filmfurther comprises a second antioxidant.
 8. The method of claim 7 whereinthe first antioxidant comprises a hindered phenolic antioxidant and thesecond antioxidant comprises a phosphite ester.
 9. The method of claim 1wherein the step of extruding the polypropylene resin occurs at atemperature of between about 220° C. and 250° C.
 10. The method of claim1 wherein the ratio of low molecular weight hindered amine lightstabilizer to high molecular weight hindered amine light stabilizer isbetween 5:1 and 9:1.
 11. The method of claim 1 wherein the ratio of lowmolecular weight hindered amine light stabilizer to high molecularweight hindered amine light stabilizer is about 7:1.
 12. The method ofclaim 7 wherein the low molecular weight hindered amine lightstabilizer, the high molecular weight hindered amine light stabilizer,the first antioxidant, and the second antioxidant are present in thepolypropylene resin in an amount less than 3% by weight.
 13. The methodof claim 1 wherein the low molecular weight hindered amine lightstabilizer has a molecular weight less than about 1000 g/mol.
 14. Themethod of claim 1 wherein the high molecular weight hindered amine lightstabilizer has a molecular weight greater than about 2000 g/mol.
 15. Themethod of claim 1 wherein the low molecular weight hindered amine lightstabilizer is bis-(2,2,6,6-tetramethyl-4-piperidinyl)-decanedioate. 16.The method of claim 1 wherein the high molecular weight hindered aminelight stabilizer ispoly[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl-4-piperidyl)imino]-hexamethylene-[(2,2,6,6-tetramethyl-4-piperidyl)imino]].17. The method of claim 1 wherein during the step of stretchingpolypropylene film, the film is stretched at a ratio of between about1:5 and 1:9.
 18. The method of claim 1 wherein during the step ofstretching the polypropylene film, the film is stretched at a ratio ofabout 1:7.
 19. The method of claim 1 wherein the stretched polypropylenefilm has a tensile strength of greater than about 250 MPa.
 20. Themethod of claim 1 further comprising the step of weaving the stretchedpolypropylene film to produce a woven polypropylene bag.